1. Field of the Invention
The invention relates to a pressure-sensing device for a vehicle wheel, more particularly to a power supply unit for a pressure-sensing device that is concealed by a tire of a vehicle wheel.
2. Description of the Related Art
U.S. Pat. No. 4,048,614 discloses a tire pressure alarm system for a vehicle wheel. The alarm system is mounted on an inner end of an air inlet valve that extends into an airtight chamber confined by a wheel rim and a tire. The alarm system is responsive to air pressure inside the chamber so as to generate a radio signal when the air pressure is at a predetermined low pressure.
U.S. Pat. No. 5,798,689 discloses a pressure gauge for a vehicle wheel. The pressure gauge includes a casing formed with an aperture and secured to the wheel rim such that the casing is concealed by a tire on the wheel rim. A signal generating device is disposed in the casing, and includes a pressure-sensing unit which generates an analog voltage signal that varies in accordance with the pressure entering the aperture, a signal converting unit which converts the analog voltage signal from the pressure-sensing unit into a digital output signal, an encoder unit which compares the digital output signal from the signal converting unit with high and low pressure limits of a normal operating pressure range and which generates a pressure signal inclusive of the digital output signal and an identification code unique to the pressure gauge when the digital output signal is not within the normal operating pressure range, and a signal transmitter circuit which transmits the pressure signal from the encoder unit wirelessly. A receiver device includes a signal receiver circuit to receive the pressure signal transmitted by the pressure gauge, a decoder unit to receive and decode the pressure signal from the signal receiver circuit, and a display unit controlled by the decoder unit so as to indicate thereon which one of the vehicle wheels is under- or over-inflated and the pressure inside the under- or over-inflated one of the vehicle wheels.
The alarm system of U.S. Pat. No. 4,048,614 and the pressure gauge of U.S. Pat. No. 5,798,689 incorporate a power source, such as a battery cell, that is also concealed by the tire of the vehicle wheel. Thus, when the stored power of the power source has been used up, there is a need to remove the vehicle wheel from the vehicle chassis, deflate the tire, remove the tire from the wheel rim, and open the alarm system or the pressure gauge before replacement of the power source can be conducted. As such, replacement of the power source is a very complicated and inconvenient task.